1. Field of the Present Invention
The present invention relates to a lithographic apparatus, a device manufacturing method, devices manufactured thereby, and to controllable patterning devices for patterning a beam of radiation.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist).
Instead of a mask, the patterning means can comprise an array of individually controllable elements, which serve to generate the circuit pattern. The patterns to be produced on the target substrate can be generated in the digital domain, and it is then necessary to convert them into corresponding, accurately defined states of the individual elements. It is desirable to be able to update the states of the elements at high rates. Electrostatic force can be used to control the positions (i.e., the states) of the controllable elements. For example, movement can be achieved by application of a suitable control voltage to one or more control electrodes arranged next to a movable element. Other control techniques can be used. The translation of the desired pattern into appropriate element states can thus comprise the generation of a plurality of analog control voltages, each one corresponding to a respective element, from digital data. It is desirable to achieve this digital to analog conversion in a manner that enables the states of all elements in the controllable patterning device to be updated at a fast rate, with low power dissipation, with low complexity, high reliability, mechanical and electrical robustness, and in a cost-effective manner.
An array can contain a large number of elements, for example up to about 2.5 million individual elements, or an even larger number. If the element states (configurations) are updated in turn, by a sequential conversion of a corresponding plurality of digital values into analog control values, then this can cause problems if a high update rate for the patterning device as a whole is required.
The conversion of digital control signals (indicative of the desired element states) to analog control voltages can be performed by a DAC (Digital-to-Analog Converter) remote from the patterning device, the analog voltages then being supplied to the individual cells by means of one or more analog input channels. Each channel can be a transmission line, and owing to the relatively high voltages being applied to the transmission lines it may not be possible to terminate them characteristically.
Arrays of individually controllable elements can be thermally sensitive. One known off the shelf DAC and amplifier combination contains a 30 V, 1 GHz op-amp with a quiescent current of about 10 mA. This gives a power output of 300 W for 1000 channels. This quiescent power is dissipated largely as heat, which can cause the array of elements to malfunction if the DAC and driving amplifier combination is located close to the array to reduce the length of the transmission line or lines. ASIC designs are typically more efficient, but still dissipate a considerable amount of power.
Another problem with DAC circuits incorporating op-amps is that the feedback loop at high frequencies typically contains a significant series inductance (corresponding to the output impedance of the op-amp). Coupled with the input of each element, which can have a large capacitive element, this could result in a LC resonator tending to cause undesirable ringing. This could prolong the settling time for the voltage supplied to each element reducing the update rate for the array.
A term “write action” can be defined as the step or series of steps involved in supplying the appropriate control voltage for a single element. According to one possible update technique, the maximum required number of write actions for a column of elements in an array of cells is equal to the maximum number of different possible voltage values (assuming those cells requiring the same voltage are supplied with that voltage at the same time). Therefore, the time allowed for programming all the elements within a column must be long enough to allow for all possible analog voltage values. For an 8-bit system, up to 256 write actions are required.
The required speed of each analog input channel depends upon the number of elements each channel supplies and the update rate of the array. A lithographic apparatus for fabricating a flat panel display can have an update rate of about 50 kHz (giving about a 20 μs update period). The array of individually controllable elements can have about 100,000 macro pixels, each consisting of about 25 elements. Each macro pixel can be controlled as a single unit. Out of the about 20 μs update period, about 10 μs is required for the mechanical settling of each element. For accuracy, the analog voltage supplied to each element must be allowed sufficient time to settle to within about 0.4% of its correct value (for an eight bit digital signal). The settling time is dependent upon the output voltage span, the slew rate of the voltage output, the maximum output current and the capacitance of the load. For a typical op-amp having a voltage span of 25 V, about 100 ns settling time is needed for each analog voltage. To program all 100,000 macro pixels within 10 μs therefore would require about 1000 analog channels.
It is undesirable to have 1000 analog input channels due to the large number of required input connections to the array. This leads to increased expense in fabricating the array. Additionally, such a large number of inputs can lead to reliability issues, high power dissipation, an increase in the required board space and problems of cross talk interference between channels.
For certain lithographic processes, the number of elements that need to be individually controlled can be much greater, for example 2.5 million elements. In future this can increase to more than 10 million elements. Additionally the required update rate is likely to increase, thereby improving the throughput for the lithographic apparatus. It is clear that by persisting with the above techniques the number of required analog inputs would quickly become unmanageable.
Therefore what is needed is a system and method utilizing a patterning means that is more effective and more efficient.